Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such as vehicles, use a middle or large-sized battery module having a plurality of battery cells electrically connected to one another because high power and large capacity are necessary for the middle or large-sized devices.
Preferably, the battery module is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle or large-sized battery module. In particular, much interest is currently focused on the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the pouch-shaped battery is lightweight, the manufacturing cost of the pouch-shaped battery is low, and it is easy to modify the shape of the pouch-shaped battery.
In order for the middle or large-sized battery module to provide power and capacity required by a predetermined apparatus or device, it is necessary for the middle or large-sized battery module to be configured to have a structure in which a plurality of battery cells is electrically connected in series or in series and parallel to each other, and the battery cells are stable against external force.
Also, the battery cells constituting the middle or large-sized battery module are secondary batteries which can be charged and discharged. Consequently, a large amount of heat is generated from the high-power, large-capacity secondary batteries during the charge and discharge of the batteries. If the heat, generated from the unit cells during the charge and discharge of the unit cells, is not effectively removed, the heat accumulates in the respective unit cells with the result that the deterioration of the unit cells is accelerated. According to circumstances, the unit cells may catch fire or explode. For this reason, a cooling system is needed in a battery pack for vehicles, which is a high-power, large-capacity battery, to cool battery cells mounted in the battery pack.
In a middle or large-sized battery pack including a plurality of battery cells, on the other hand, the deterioration in performance of some battery cells leads to the deterioration in performance of the entire battery pack. One of the main factors causing the non-uniformity of the performance is the non-uniformity of cooling between the battery cells. For this reason, it is required to provide a structure to secure the uniformity of cooling during the flow of a coolant.
As shown in FIG. 1, a conventional middle or large-sized battery pack 20 is configured to have a structure in which unit cells are arranged in a width direction of the battery pack 20 in an upright fashion to constitute a battery module layer 10, and two or more battery module layers 10 are mounted in a pack case 15 such that the battery module layers 10 are stacked in a height direction of the battery pack 20.
In this structure, a coolant, introduced through a coolant introduction part 11 formed at the top of the pack case, sequentially passes through flow channels defined between the unit cells arranged in the vertical direction to cool the unit cells, and is then discharged out of the pack case through a coolant discharge part formed at the bottom of the pack case.
In this structure, however, temperature deviation between the battery module layers is generated, and the coolant flow channel is increased with the result that pressure difference between the unit cells is generated, whereby it is difficult to uniformly cool the battery cells.
Consequently, there is a high necessity for a technology to fundamentally solve the above-mentioned problems.